Reviewer #1 (Public Review):
Elbaz-Hayoun et al. investigate the role of macrophages in the gliotic response of retinal Müller glia and photoreceptor cell death. Monocytes (a precursor of macrophages) were isolated from age-related macular degeneration (AMD) patients. When injected into light-damaged retinas, a reduction in the number of photoreceptors and ERG b-wave strength (evidence of abnormal photoreceptor function) was observed. The authors reasoned that macrophages generated from the injected monocytes might be responsible for the retinal damage. To test this hypothesis, macrophage subtypes were generated from AMD-derived human monocytes and injected into light-damaged mouse eyes. Interstingly, only the human hM2a macrophage subclass mimicked the retinal degeneration of monocyte injection in mouse retinas. Similarly, human M2a (hM2a) cells cultured on mouse retinal explants and even serum-free hM2a culture supernatant were sufficient to induce photoreceptor apoptosis. These effects were not observed with hM1 cells. To identify possible diffusible factors responsible, proteins present in hM2a and hM1 culture supernatants were identified. Nine cytokines were found at higher levels in the hM2a supernatant, and three of these were ligands for the C-C chemokine receptor CCR1. The authors confirmed CCR1 expression in the retina, which was predominantly detected in Müller glia. Importantly, Müller cell expression of CCR1 in the mouse retina was significantly increased following light damage. In contrast, CCR2 and CCR5 levels were unchanged in Müller cells. The increase in CCR1 expression, gliosis, and photoreceptor death was also observed in the rd10 mouse model of retinitis pigmentosa. Inhibiting CCR1 activity in light-damaged eyes using the drug BX471 had impressive effects. Müller activation and photoreceptor cell death were reduced and ERG b-wave levels were partially recovered - clearly indicating a role for CCR1 in retinal degeneration. Additional evidence was provided suggesting that CCR1 activation in M2a macrophages might also play a role in stimulating the movement of other macrophages into the retina and activating retinal microglia, which migrate to the ONL. These data identify a new link between cells of the immune system and those within the retina which contribute to the progression of retinal degeneration.
The data mostly support the conclusions of this paper. However, additional controls need to be added to some experiments.
Concerns:
1) To determine the effect of diseased monocytes on retinal health, light-injured mouse retinas were injected with monocytes isolated from AMD patients (Figure 1 - figure supplement 1). This resulted in a reduction in photoreceptor number and ERG b-wave amplitude. However, the light-injured control eye was injected with PBS only, so no cells were present. The reasoning for using this control was not provided. The appropriate injection control would include monocytes isolated from non-AMD patients. This control should be performed side-by-side with cells from AMD patients.
2) The authors hypothesize, from the experiments presented in Figure 1 - figure supplement 1, that the injected monocytes generated macrophages in the retina, which were responsible for the observed neurotoxicity (Lines 143-145). However, no direct evidence was presented. This idea should be tested in vivo. This could be done by injecting tracer-labeled human AMD-derived monocytes into light-injured mouse retinas. If the authors' hypothesis is true, collected retinas should contain tracer-labeled cells that express macrophage markers. Tracer-labeled M2a macrophage cells should be present since subsequent experiments identify this subclass as being associated with retinal cell death.
3) Photoreceptor number and b-wave amplitudes were measured in light-injured retinas injected with one of four macrophage cell types generated from human AMD-derived monocytes. The authors conclude that only injection of M2a cells reduced photoreceptor number and b-wave amplitudes (Figure 1C, E). This may be true, but it is difficult for the reader to make a conclusion (especially in Fig. 1E) due to the large error bars and five different traces overlapping each other. To make these results easier to interpret, graph control cells with only one experimental sample (cell type) at a time.
4) Most injected macrophages were located in the vitreous. In the case of M2a cells, the authors note that "several of the cells migrated across the retinal layers reaching the subretinal space" (Lines 167,168). One possible explanation for why M0, M1, and M2c macrophages did not induce retinal degeneration is that they did not migrate to the subretinal space and around the optic nerve head. Supplementary figures should be added to demonstrate that this is not the case.
5) Figure 1 - figure supplement 2: Panel A, B cells were stained with CD206 to demonstrate the presence of M2a macrophages (panel B). The authors conclude that panel A contains M1 and panel B contains M2a cells. The lack of CD206 expression illustrates that panel A cells are not M2a macrophages but do not demonstrate they are M1 macrophages. A control using an M1 cell marker is necessary to show that panel A cells are M1 and M1 cells are not detected in M2a cultures.
6) Ex vivo, apoptotic photoreceptor and RPE cells are observed when cultured with M2a macrophages (Figure 2). Do injected M2a cells also induce apoptosis of RPE cells in vivo? This is important to establish that retinal explants are a good model for in vivo experiments.
7) Reactive oxygen species (ROS) production was measured to determine if M2a cell-mediated neurotoxicity was due to oxidative stress. It is concluded that a ROS increase is partly responsible (Line 218). The data do not support this conclusion. ROS was detected in cultured M2a macrophages. More importantly, however, there was no increase in oxidative damage in vivo. The in vivo and cell culture results contradict each other so no conclusion can be made. The lack of in vivo confirmation weakens the argument that ROS drives M2a neurotoxicity. Text suggesting a role for ROS in neurotoxicity should be appropriately edited (Lines including 218, 244, 401,406,481).
8) The authors ask if the photoreceptor cell death is cytokine-mediated. Multiple cytokines were enriched in M2a-conditioned media. Of particular interest were CCR1 ligands MPIF1 and MCP4. The implication is that these two ligands mediate the M2a macrophages to photoreceptor cell death through CCR1. However, there is no attempt to show that either MPIF1 or MCP4 are present in vivo, or are sufficient to induce the retinal response observed. This could be demonstrated by injection of MPIF1 or MCP4. Evidence that either ligand phenocopies M2a macrophage injection would be direct evidence that CCR1 ligands activate the retinal response. Furthermore, co-injection with BX174 should block the effect of these ligands if they work through CCR1.

